stagesprophase

Stages of prophase (often discussed in the context of mitosis) describe the early steps by which chromatin condenses and the cell prepares for chromosome segregation. In traditional accounts, prophase is subdivided into early prophase and late prophase, with transitions into prometaphase as the envelope breaks down and spindle interactions mature.

Early prophase involves condensation of chromatin into visible chromosomes, each chromosome consisting of two sister chromatids held at the centromere. The nucleolus disappears as ribosomal RNA synthesis declines, and the centrosomes move toward opposite poles, organizing the initial spindle apparatus from microtubules and other proteins. The nuclear envelope remains intact during this stage.

Late prophase marks further progression toward chromosome segregation. The nuclear envelope disperses or breaks down, allowing spindle microtubules to interact directly with chromosomes. Kinetochores form at centromeres and attach to microtubules, enabling chromosomes to begin moving toward the cell center. The spindle apparatus becomes fully organized, with poles established and motor proteins guiding chromosome alignment and movement.

In many textbook treatments, events of late prophase are described as part of prometaphase, making the boundary between prophase and prometaphase transitional rather than sharply defined. Prophase in meiosis is distinct: prophase I includes sub-stages leptotene, zygotene, pachytene, diplotene, and diakinesis, during which homologous chromosomes pair and may exchange genetic material.

Overall, the stages of prophase set the stage for accurate chromosome alignment at metaphase and subsequent equal segregation into daughter cells.